This trophy is titled "Irresistible" and it's for hitting the opponent with Tager's "Spark Bolt" more than six times in one round. This move uses the electric meter that is shown below the picture of your character. It refills automatically, however you can speed the process by tapping back and doing a quarter-circle backwards motion. Tager should do a pose and a portion of his meter will fill. It's similar to the "shoryuken" movement only backwards.
The move itself is executed by doing a half circle starting away from yourself going towards the opponent. Looking at the number pad, this would mean 63214 followed by D (X on default controls). Make sure to do this 6 times in one round, not match, and you should have your trophy in a few minutes.